1. Field of the Invention
The present invention relates to exhaust systems.
2. Description of the Background Art
Exhaust fumes generated during the combustion of gasoline or diesel flow through exhaust systems of internal combustion engines. They contain carbon monoxide, carbon dioxide, nitrogen oxides, sulfur oxides, ammonium compounds and water vapor, to name a few. If the exhaust fumes drop below the dew point, particularly in the start and partial load range, a liquid condensate forms inside the exhaust system, with a pH-value of between 6.5 and 2.5. This condensate causes corrosion of the steel material that the exhaust system is made of.
Year after year, the automobile manufacturers demand a higher product life of the exhaust system. The objective is lifetime serviceability. Thus, the manufacturers of exhaust systems are forced to switch to stainless steels instead of surface-passivated steels, for example, aluminated steel. However, even stainless steel is subject to corrosion resulting from the combined effects of acid and heat. For this reason, the sheet metal for the manufacture of the pipes and housings must be of a considerable thickness. This makes it harder to work with, and increases both the weight and the cost.
It is known from container-building as well as shipbuilding to protect steel parts from corrosion by using sacrificial anodes, which essentially contain zinc and/or magnesium. A typical example are storage tanks of hot-water systems, whereby the storage tanks are completely filled with water so that the zinc or magnesium ions released by the sacrificial anodes can reach every part of the tank. In the corrosion protection of the body of a ship, seawater makes it possible for the ions to migrate to all parts of the ship's hull that are subject to corrosion.
It is suggested in ES 2073969 A to protect the entire chassis of a motor vehicle against corrosion with a sacrificial anode. The anode corrosion protection is to include all the components of the exhaust system that are connected to the chassis. This would protect the exhaust system from external corrosive attacks, for example, melted road salt, but not, however, from the internal corrosive attack by the condensate.
In actuality, the use of sacrificial anodes in the construction of automobiles did not prevail. Instead, the chassis is made of galvanized sheet metal.
Exhaust systems, on the other hand, are not made of galvanized sheet metal because using galvanized sheet metal to make exhaust pipes and housings is extremely problematic. As previously mentioned, stainless steels with a high content of chromium and nickel are used instead, or else ordinary steels coated with an aluminum alloy. When exposed to air, the aluminum coatings form an oxide layer, which effectively wards off corrosion attacks. However, as soon as the aluminum coating is damaged in one place, for example, when the steel is bent, punched, molded, or welded, the corrosion continues to advance unimpededly.